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AIAC12 Twelfth Australian International Aerospace Congress19 – 22 March 2007

Corrosion and corrosivity monitoring system

Paul Dietrich1, Russ BraunlingHoneywell International, 3660 Technology Drive, Minneapolis, MN, USA 55418

ABSTRACT

Honeywell International has developed and flight-tested a Corrosion and Corrosivity Monitoring System(C2MS). The C2MS detects galvanic corrosion in the main gearbox feet fasteners of helicopters. In addition,it monitors the environmental conditions inside the main floorboard compartment to determine the need forstructural maintenance. The C2MS sensor on a main gearbox feet fastener sends a small electrical signalthrough the fastener and housing to measure the conductivity of the assembly. The measured conductivityvalue is used to determine if galvanic corrosion is present in the fastener assembly. The floorboardcompartment sensors use a surrogate metal coupon to measure the corrosivity of the environment. Theinformation from this sensor is used to recommend an extension to the calendar-based maintenance schedule.Fleet-wide information can be gathered by the system. The C2MS uses two Data Collection Units (DCUs) tostore the corrosion data: one for the main gearbox feet fasteners and one for the main floorboardcompartment. The DCU design addresses the issues of long battery life for the C2MS (greater than 2 years)and compactness. The data from the DCUs is collected by a personal digital assistant and downloaded to apersonal computer where the corrosion algorithms reside. The personal computer display provides thelocation(s) of galvanic corrosion in the main gearbox feet fasteners as well as the recommended date forfloorboard compartment maintenance. This paper discusses the methodology used to develop the C2MSsoftware and hardware, presents the principles of the galvanic corrosion detection algorithm, and gives thelaboratory and flight test results that document system performance in detecting galvanic corrosion (detectionand false alarm rate). The paper also discusses the benefits of environmental sensors for providing amaintenance scheduling date.

Keywords: Corrosion, galvanic corrosion, corrosivity, condition-based maintenance, low power, sensorsystem

1. INTRODUCTION

Maritime aircraft operate in a particularly hostile, corrosive environment. Salt is always present on ship deckand can be encountered at altitudes up to 1500 feet above sea level. Since aircraft materials are primarilychosen for their lightness and strength, they are prone to corrosion. In addition, fastener assemblies in theaircraft require steel bolts for strength that are screwed into lightweight magnesium casing. This use ofdissimilar metals in the main gearbox fastener assembly is a prescription for galvanic corrosion. Galvaniccorrosion can cause rapid damage to flight-critical structures, which has a detrimental effect on safety andoperational capability. To minimize the effects of corrosion, maintenance personnel must regularly scheduleexpensive and time-consuming inspections. Currently, maintenance uses visual inspections, man-in-the-loopinspections with various types of non-destructive evaluation (NDE) equipment, and regularly scheduled stripand search inspections. Clearly, an autonomous corrosion monitoring system is needed to reduce manloading and provide reliable early warning corrosion information suitable for condition-based corrosionmaintenance.

1 [email protected]; phone 1 612 951-7851; fax 1 612 951-7438; www.honeywell.com

mailto:[email protected]


Honeywell International (United States) partnering with Avonwood Developments LTD (United Kingdom)has developed and flight tested a Corrosion and Corrosivity Monitoring System (C2MS). The C2MScontinuously monitors two corrosion prone areas in helicopters the fastener assembly for the main gearboxhousing, and the main floorboard compartment. The C2MS provides the only in situ galvanic corrosiondetection system that we are aware of that can continuously monitor the main gearbox fastener assembly.The C2MS uses corrosive environment sensors (CES) in the floorboard compartment to provide a moreaccurate estimation of the date to inspect the compartment than that obtained from a calendar-basedmaintenance system.

2. SYSTEM OVERVIEW

The C2MS constantly monitors critical equipment and corrosion prone areas in helicopters to provideinformation for condition-based maintenance. The C2MS performs two corrosion sensing functions. Itdetects galvanic corrosion in the main gearbox feet fasteners and monitors environmental conditions insidethe main floorboard compartment of helicopters to determine the need for structural maintenance. Earlydetection of galvanic corrosion in the main gearbox feet fasteners ensures flight safety and mission readinesswhile eliminating unnecessary fastener inspections. The C2MS sensors on the main gearbox feet fastenersend a small electrical signal through the fastener and housing to measure the conductivity of the assembly.The conductivity information is used to determine whether galvanic corrosion is present in the fastenerassembly. The floorboard compartment sensors use a surrogate metal coupon for the measurement of thecorrosivity of the environment. Maintenance intervals can typically be extended by measuring the actualcorrosivity of the environment rather than using worst-case-based calendar inspections. Figure 1 shows anoverview of the C2MS.

Figure 1: Overview of C2MS operation

The C2MS consists of two data collection units (DCUs), a personal digital assistant (PDA), and corrosionanalysis and display software that run on a PC. Each DCU is composed of corrosion sensors and electronicsfor autonomously collecting and storing corrosion data. The PDA is used to gather corrosion data from theDCUs and transfer the data to a PC for analysis, display, and archiving. Figure 2 shows the DCU andattached sensors. Each DCU is configured for either galvanic corrosion detection (eight sensors) or corrosionenvironment sensing (three sensors).

5th DSTO International Conference on Health & Usage Monitoring


Figure 2: Data Collection Unit

3. CORROSION SENSOR METHODOLOGY

The C2MS is a battery operated sensor system that detects galvanic corrosion in the main gearbox feetfasteners and constantly monitors the corrosive environment inside the floorboard compartment. Thegalvanic corrosion sensor (smart bolt cap or smart washer) and the CES perform their sensor functionsindependently. The galvanic corrosion sensor has a mission life greater than four years and the CES has amission life of from two to four years, depending on the environment inside the floorboard compartment.The smart bolt cap and smart washer sensor follow the same principle of operation for detecting galvaniccorrosion; however, they are attached in a different manner in the fastener assembly. The operation of thesmart bolt cap and CES are discussed in the following subsections.

3.1 Smart bolt cap sensorGalvanic corrosion occurs when two dissimilar metals are in direct contact with each other and moisture ispresent. In a helicopter, the magnesium transmission case is bolted to the helicopter with a steel bolt and nut.When galvanic corrosion occurs, the magnesium case becomes the anode and the steel bolt the cathode withmagnesium the sacrificial metal in the galvanic corrosion cell.

Because galvanic corrosion can be very aggressive and must be avoided, a standard protective systemprevents direct contact between the dissimilar metals and prevents moisture penetration between the metals.The system includes protective paint on the magnesium case, protective coating (e.g., cadmium on the steelbolt and nut), and a protective sealant on and around the exposed bolt head. The paint helps prevent electricalcontact between the bolt head and the magnesium case. The cadmium coating helps reduce galvanic action.

The smart bolt cap sensor has a metal sensing ring that is periodically energized by the C2MS battery. Undernormal conditions, when the fastener's protective system is intact and no galvanic corrosion is present, thesensing ring is electrically insulated from the metal structure. In this case, no current can flow from thebattery through the conductive ring to chassis ground. This condition results in a measured resistance valuealong this path of several hundred mega ohms. If a failure occurs in the protective system, i.e., the sealant andprotective coating have broken down, and moisture has penetrated into the assembly initiating galvaniccorrosion, then there must be a conductive path from the battery, through the sensing ring to chassis ground.This condition results in a measured resistance value of several hundred kilo ohms. This drop in resistivevalue indicates galvanic corrosion. Note that moisture must penetrate the assembly to create a conductive pathto ground and thus make the low resistance measurement possible. However, moisture in the assembly cancome and go relatively quickly depending upon environmental conditions. C2MS deals easily with thissituation by measuring the resistance value several times a day, then latching to a galvanic corrosionDETECT whenever the resistive change is measured. (In actuality, multiple resistive measurements and a



voting scheme are used during a single interrogation cycle.) Once a DETECT is declared it remains on untilreset.

The smart bolt cap and smart washer sensor are patented in both the United States and the United Kingdom.

3.2 Corrosive environment sensorRemoving the helicopter floorboards and performing compartment maintenance requires considerable labor(man loading) and causes significant down time, affecting mission readiness. Floorboard compartmentmaintenance is typically performed on a calendar-based schedule. Note that floorboard compartment sealantsand coatings are frequently upgraded, which tends to lengthen the schedule. While extending the timebetween maintenance cycles is, in general, good, it also increases the risk that the floorboard compartmentwill experience excessive damage before maintenance is performed. The CES is designed to reduce the manloading requirements and overall maintenance costs by providing a condition-based maintenance schedule forfloorboard removal. While mission readiness and other requirements do not afford the luxury of maintainingeach helicopter at the optimum time, it is conjectured that fleet-wide condition information can be used toprovide the optimum low-cost maintenance solution for the fleet.

The CES uses a metal "coupon" to constantly measure the corrosive environment inside the floorboardcompartment. The CES does not detect or determine the location of metal corrosion in the compartment'sstructure. This means the CES scheduling methodology is completely inferential, i.e., the worse theenvironment in the compartment over the longer period of time, the more need there is to performmaintenance. We have selected stainless steel as the coupon material rather than the metal used in thefloorboard compartment. This enables a higher sensitivity measurement of the environment inside thecompartment than that obtained using aluminum which is more resistant to corrosion. With the stainless steelcoupons our algorithm uses the principles of electrical resistance NDE (resistance increases as metalthickness decreases) to continuously estimate the corrosion rate and the cumulative metal loss of the coupon.The cumulative metal loss for the stainless steel coupon is compared against the cumulative metal lossexpected from ISO9223 corrosivity categories C1-C5 for stainless steel. A proprietary algorithm is then usedto estimate the expansion or contraction from the calendar based maintenance date. The CES also contains atemperature and humidity sensor that can be used to calculate the Time of Wetness (TOW) inside thecompartment. Our original intent was to use TOW as collaborative information with the electrical resistancesensor. However, we have yet to see any real advantage from these additional sensors and our plan is toeliminate the temperature and humidity sensors from future C2MS units.

The approach used for the CES is patent pending in the U.S. and a foreign patent application has also beenfiled.

4. FLIGHT TEST SYSTEM

4.1 System hardwareThe system hardware consists of Data Collection Units (DCUs) and a set of attached sensors. Two DCUs arerequired to instrument a helicopter, one configured with corrosive environment sensors and one configuredwith galvanic corrosion sensors. Figure 3 shows a DCU with all three corrosive environment sensors andFigure 4 shows a DCU with all eight smart bolt cap sensors.

The other form of the galvanic corrosion sensor is for helicopters with washers in the fastener assembly. Forthis case, a smart washer sensing ring is attached to the outer rim of the Equipment Manufacturer (OEM)washer. Figure 5 shows a smart washer sensor.



Figure 3.DCU and corrosive environment sensors Figure 4. DCU and smart bolt cap sensors

The DCU power is enabled using a reed switch, which consists of two ferromagnetic reeds, hermeticallysealed in a gas filled capsule. When these reeds are introduced to a magnetic field, they assume oppositepolarities and attract each other closing the contacts. When the magnetic field is reduced beyond the springforce of the reeds, the contacts spring open. The magnetic field is supplied by an internal magnet that slidesinto place during the installation process.

The DCU electronics are sealed in an aluminum enclosure with a silicone elastomer to reduce corrosion andvibration concerns. EMI radiation from the hardware was mitigated by careful use of components with lowradiation properties. The grounded aluminum enclosure also shields against EMI transmission both to andfrom the helicopter.

Figure 5: Smart washer sensor

Figure 6: Helicopter vibration exposure

Vibration testing was performed on the hardware using a sinusoidal vibration with broadband backgroundsuperimposed. Four different frequencies corresponding approximately to the fundamental, blade passageand 1st and 2nd harmonics were tested (f1, f2, f3, and f4). Each axis was tested at these frequencies for a periodof 1 hour for a total of 12 hours testing. Figure 6 shows the typical vibration exposure for the helicopter.Shock testing was done using the shock response spectra shown in Figure 7.



Temperature testing was performed by rampingfrom room temperature to -40°C where itremained for 4 hours to stabilize the temperature.It then maintained operation for 30 minutes.Next, the temperature was ramped to 32°C andoperation was maintained for 30 minutes. Thetemperature was ramped to 60°C where itremained for two hours to stabilize thetemperature. It then maintained operation for 1hour. Finally, the temperature was ramped backto room temperature and the cycle was repeated10 times.

The DCU communicates to a PDA with a wiredserial link.

Figure 7: Test shock response spectra

The two DCUs are bussed together using a shielded twisted pair cable with a connector installed for a singlepoint data download to the PDA. No specific time is set for system download; however, data should becollected as often as practicable. The sooner the conditions for galvanic corrosion are detected, the quicker amaintenance check can be performed. At the time of download, all data collected since the previousdownload is transmitted.

4.2 Sensor locationsThe DCUs are mounted in locations that are convenient for the sensors. Since no access to the DCU isrequired for operation or data download, they do not need to be easily accessible. Error: Reference source notfound shows one DCU mounted with epoxy to a roof strut. The corrosive environment sensors are locatedunder the floorboards and are secured to the helicopter skin with epoxy. Error: Reference source not foundshows one of the corrosive environment sensors with the protective yellow tape still in place. This tape isremoved after the corrosion preventative spray is applied to the compartment. The galvanic corrosion sensortakes on two forms: the smart bolt cap sensor and the smart washer sensor. The smart bolt cap sensor is usedwhen no washer is present in the fastener assembly. It attaches to a recess in the bolt head by tightening a nuton the sensor. Figure 10 shows the smart bolt cap sensor installed on two of the main gearbox feet fasteners.The smart washer sensor replaces the OEM washer in the assembly and is shown in Figure 11.

Figure 8: DCU mounted in a helicopter

Figure 9: Corrosive environment sensor mounted beneath floorboards



Figure 10: Smart bolt cap sensor mounted on main gearbox feet fasteners

Figure 11: Smart washer sensor mounting location

5. FLIGHT TEST RESULTS

Corrosion algorithms and display software reside on the PC to which the corrosion data is downloaded. TheC2MS display screen is shown in Figure 12. In the middle left portion of the screen the helicopter TailNumber is selected. Information in the top portion of the screen indicates whether galvanic corrosion hasbeen detected in any of the eight gearbox feet fasteners in the helicopter. For the case shown, all eightgearbox fastener assemblies are green, indicating that galvanic corrosion is not present. If galvanic corrosionis detected, one or more of the green-filled circles will turn red. The information in the center of the screenindicates the recommended extension or contraction in the calendar-based maintenance schedule. Here, therecommended extension is 12 weeks, our current maximum extension time. The calculated date for theinspection is also shown in the middle of the figure. The button (Floorboard Compartment Environment) onthe middle right of the screen can be used to call another screen that displays detailed information on theenvironment measured inside the floorboard compartment.

During an 18-month flight test in the United Kingdom using three helicopters, two galvanic corrosiondetections occurred. The two galvanic corrosion detections were verified by manual inspection of thefastener assemblies. There were no false alarms over the 18-month test period in the United Kingdom.During two flight tests in the United States, one of 13-month duration and the second of 14-month duration,no galvanic corrosion detections occurred. A third flight test of 16 months in the United States had twogalvanic corrosion detections. The first detection was recorded but not inspected. When the inspection wasperformed two months later another detection was recorded. The inspection verified a break in the sealantalong with the presence of moisture. As expected, there were no false alarms during the tests in the UnitedStates.

During a 14-month and a 16-month helicopter test in the United States, the CES reported unusually low levelsof environmental corrosivity inside the floorboard compartment and recommended the maximum extensiontime of three months before maintenance is performed. For the 16-month test the floorboards were removedat the end of the test; however, for the 14-month test the floorboards were removed to perform maintenanceafter 10 months. In both cases, the maintenance team reported that the floorboards were "clean" andmaintenance was unnecessary. Checks with the flight data logs indicated that both helicopters had only a fewhours flying time over salt water and were basically in a low salt environment over their entire test periodsfurther verifying the CES recommendations.

The C2MS does not currently provide a summary display of fleet-wide corrosion conditions as shown inFigure 13. However, we anticipate adding this feature to future versions of the C2MS.



Figure 12: Graphical User Interface (GUI) main display. Figure 13. Fleet summary.

6. CONCLUSIONS

The C2MS detects galvanic corrosion in fastener assembles and monitors hard-to-reach areas to determinewhether corrosion maintenance is needed. While the C2MS was developed for and is being tested on rotaryaircraft, the technology is directly applicable to several other platforms including fixed wing aircraft andships. We are not aware of any other sensor that can unobtrusively monitor fastener assemblies for galvaniccorrosion. In addition, because of the simplicity of the principle used in the detecting galvanic (measuring alarge decrease in resistance), the system is virtually false-alarm proof.

The goal of the CES is to provide better information in scheduling maintenance than can be obtained using acalendar-based maintenance system. Scheduling is accomplished using onboard sensors to measure corrosiveactivity. This approach provides the maintenance engineer with "a pair of eyes" in difficult locations thatshould be a valuable tool in scheduling maintenance.

Testing of the C2MS through this date has produced outstanding results; however, more testing is required toform a statistically significant data base to further evaluate system performance. Another difficulty facing theC2MS is one that faces all condition-based maintenance systems—lack of documented corrosion maintenancecosts to establish and prove a value proposition for the maintenance system. We can only hope that in thefuture studies will document maintenance costs and provide a rationale for condition-based maintenancesystems.



ACKNOWLEDGEMENTS

The Office of Naval Research helped fund the development and flight testing for the Corrosion andCorrosivity Monitoring System (C2MS). We appreciated the support of the Naval Air Systems Command(NAVAIR) and Aviation Applied Technology Directorate (AATD) for use of their helicopters and personnelfor the data collection effort. Avonwood Developments Ltd. is acknowledged for the design anddevelopment of the smart bolt cap and smart washer sensor.



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